1. Field of the Invention
The present invention relates to a cross dichroic prism that color-synthesizes R (red) color light, G (green) color light and B (blue) color light having entered the prism separately and outputs color synthesized light. It also relates to a projection display apparatus that uses such a cross dichroic prism in color synthesis.
2. Description of Related Art
FIG. 11 is a plan view showing a structure adopted in a projection display apparatus in the related art and FIG. 12 is a sectional view showing a structure of a cross dichroic prism 109 utilized as a color synthesis optical system in the projection display apparatus.
Light emitted from a light source 101 enters a cross dichroic mirror 102. The cross dichroic mirror 102 includes a dichroic mirror 102B that reflects B (blue) light and a dichroic mirror 102RG that reflects R (red) light and G (green) light. The dichroic mirrors are disposed at right angles to each other. Thus, the light from the light source 101 is separated at the cross dichroic mirror 102 into B light and mixed light containing R light and G light advancing along directions opposite from each other.
The B light enters a deflecting mirror 103 where it changes its advancing direction to enter a B-light polarization beam splitter 106B. The B-light polarization beam splitter 106B reflects the B light having entered therein at its polarization splitter unit and the reflected B light is directed to enter a reflective light valve 107B for B light. The mixed light containing the R light and the G light enters a deflecting mirror 104 where it changes its advancing direction and the mixed light the n enters a G-light reflecting dichroic mirror 105. At the G-light reflecting dichroic mirror 105, the mixed light is separated into reflected G light and transmitted R light. The G light and the R light having undergone the color separation and having exited the G-light reflecting dichroic mirror 105 separately then enter polarization beam splitters 106G and 106R provided in correspondence to the respective colors of light. The G light and the R light having entered the respective polarization beam splitters 106G and 106R are reflected at polarization splitter units and the G light and the R light having exited the polarization beam splitters are directed to enter reflective light valves 107G and 107R for G color light and R color light, respectively.
The light beams corresponding to the individual colors having entered the B-color light reflective light valve 107B, the G-color light reflective light valve 107G and the R-color light reflective light valve 107R are modulated based upon color signals and mixed light beams containing modulated light and unmodulated light are reflected at the respective reflective light valves toward the polarization beam splitters 106B, 106G and 106R. After the mixed light beams, each containing the modulated light and the unmodulated light, enter the polarization beam splitters 106B, 106G and 106R respectively, the modulated light components alone are transmitted and extracted as analyzed light beams.
The analyzed light beams then enter a cross dichroic prism 109 through different incident surfaces. The cross dichroic prism 109 includes an R light reflecting dichroic film 109R and a B-light reflecting dichroic film 109B disposed therein at substantially right angles to each other. The R light and the B light having entered the cross dichroic prism 109 are reflected at the R light reflecting dichroic film 109R and the B-light reflecting dichroic film 109B respectively, whereas the G light having entered the cross dichroic prism 109 is transmitted through the dichroic films 109R and 109B. As a result, the R light, the B light and the G light become color synthesized at the cross dichroic prism 109 and they are emitted as synthesized light. The synthesized light having exited the cross dichroic prism 109 is projected via a projection lens 110 onto a screen (not shown) on which a projected image is formed.
FIG. 12 is a sectional view, showing the structure of the cross dichroic prism 109 constituting the color synthesis optical system. The method that may be adopted when manufacturing the cross dichroic prism 109 is first explained. Four right angle isosceles triangle prisms 109-1, 109-2, 109-3 and 109-4 having sectional shapes identical to one another are procured. B-light reflecting dichroic films 109B are formed at side surfaces of the prism 109-1 and the prism 109-3, each at one of the side surfaces forming the right-angle vertex of the corresponding prism. Next, the prism 109-1 and the prism 109-2 are bonded via an adhesive layer 109S so as to bond the surface of the prism 109-1 where the dichroic film has been formed to a side surface of the prism 109-2 and likewise, the prism 109-3 and the prism 109-4 are bonded via an adhesive layer 109S so as to bond the surface of the prism 109-3 where the dichroic film has been formed to a side surface of the prism 109-4. As a result, two prism pairs are formed.
Next, the hypotenuse surfaces facing opposite the right angle vertices of the two prism pairs are polished so as to achieve a desired level of flatness. Then, an R-color light reflecting dichroic film 109R is formed at the polished surface of the prism pair made up of the prisms 109-1 and 109-2, the surface at which the R-color light reflecting dichroic film 109R has been formed is bonded to the polished surface of the prism pair made up with the prism 109-3 and 109-4, and thus, the cross dichroic prism 109 is formed. The two prism pairs are bonded via an adhesive layer 109S by ensuring that the B-light reflecting dichroic films 109B formed at the two prism pairs are set so as to form a single flat surface.
In the cross dichroic prism 109 adopting the basic structure shown in FIG. 11, R light enters the prism 109-2, B light enters the prism 109-3 and G light enters the prism 109-4 and synthesized light exits through the prism 109-1, as shown in FIG. 12. Cross dichroic prisms adopting such a structure are disclosed in the following patent reference literatures.    Patent reference literature 1: Japanese Laid Open Patent Publication No. H10-311907    Patent reference literature 2: Japanese Laid Open Patent Publication No. H10-311908    Patent reference literature 3: Japanese Laid Open Patent Publication No. H10-311909    Patent reference literature 4: Japanese Laid Open Patent Publication No. 2002-189109